V. Yu. Borovkov

Nature of the sites of dissociative adsorption of dihydrogen and light paraffins in ZnHZSM-5 zeolite prepared by incipient wetness impregnation

A DRIFT study of ZnHZSM-5 zeolites with Si/Al ratios of 15 or 41 and a Zn loading of 0.8 wt% revealed a high thermal stability of bridging OH groups that was practically the same as in the pure hydrogen forms. It was concluded that the incipient wetness impregnation of NH4ZSM-5 zeolite with zinc nitrate and the subsequent high-temperature treatment results only in a minor amount of ion exchange. A considerable part of the modifying zinc forms nanometric ZnO clusters inside the channels of the zeolite. The use of the low-temperature adsorption of dihydrogen as a probe indicated the appearance, after high-temperature vacuum pretreatment, of three different Lewis acid sites connected with coordinatively-unsaturated Zn2+ ions. The strongest Lewis sites, with an H–H stretching frequency of adsorbed molecular hydrogen of 3940 cm−1, dissociatively adsorbed hydrogen, methane and propane at both room and elevated temperatures. These sites are represented either by Zn2+ ions on the walls of the main channels of the zeolite (α sites according to Mole et al.) or by Lewis-base pairs on the surface of nanometric clusters of zinc oxide.

Diffuse reflectance IR-spectroscopy evidence of the unusual properties of platinum in Pt/Mg(Al)O catalysts for the selective aromatization ofn-alkanes

Diffuse reflectance IR spectra of CO adsorbed on Pt/Mg(Al)O catalysts were studied at room temperature and different surface coverages of metallic Pt. They show the existence of two types of metallic Pt particles. One type has a normal behavior toward the adsorption of CO, which is probably related to the lower dispersion of Pt. The other type probably corresponds to smaller negatively charged clusters like those found for Pt-loaded alkaline and alkaline-earth forms of zeolites. They are characterized by the very low frequency of the singletone corresponding to the linear form of adsorbed CO and high thermal stability of the bridging complexes of CO with metallic Pt. Such by metal-support interaction modified platinum is likely responsible for high activity and selectivity of Pt/Mg(Al)O catalyst inn-alkanes aromatization.

Diffuse reflectance IR study of noble metals supported on basic carriers. Part I : Pt supported on Al-Mg hydrotalcite

The state of metallic Pt supported on alumina-magnesia hydrotalcite was investigated using diffuse reflectance FT-IR spectra of adsorbed CO and terminal Pt hydrides formed upon hydrogen dissociative chemisorption on metal at room temperature. On hydrotalcite surface metallic Pt exists in three different forms, such as neutral and negatively charged metallic particles as well as slightly negatively charged clusters encapsulated between layers of support. Negative charging on Pt particles results in decrease of singleton frequency of linearly adsorbed CO by 70 cm-1 and frequency of Pt-H bond vibration by 50-100 cm-1. It was also established that negatively charged Pt is less sulfur resistant than neutral Pt particles.

On the role of zinc oxide nanometric clusters in preparation of ZnNaY zeolite by ion exchange

Dehydration of Zn2+ exchanged NaY zeolite was studied by means of DRIFT spectra of adsorbed dihydrogen and carbon monoxide as molecular probes. Unlike alkaline earth forms of faujasites dehydration of ZnY zeolite at moderately high temperatures results in the formation of bridging protons and nanometric ZnO or mixed hydroxylated ZnO microclusters instead of bridging hydroxy groups and MeOH+ cations. After subsequent dehydration at above 573 K the zinc oxide clusters interact with acidic protons resulting in Zn2+ ions localized at SII sites. These sites adsorb hydrogen in its molecular form. Heterolytic dissociative adsorption does not take place even at elevated temperatures.

Characterization of the Pt-C bond of CO adsorbed by small Pt particles in a NaX zeolite by IR diffuse reflectance spectroscopy

The unusual properties of small (1–2 nm) Pt particles within a zeolite support are documented by diffuse reflectance infrared spectroscopy of the metal-carbon bond of linearly adsorbed CO. The wave number of the metal-carbon stretching frequency is found to be about 60 cm−1 higher compared to larger particles (2–3 nm) or to the bulk metal. The stronger bond is attributed to a negative charge on the small metal particles.

Infrared and adsorption study of strong metal-support interaction in diluted platinum-alumina catalysts

Diffuse reflectance IR spectroscopy along with H2 and CO adsorption measurements were applied to the study of 0.3% Pt/-γ-Al2O3 catalyst precalcined in an air flow at 1023 K and then reduced at 773 and 973 K in hydrogen. Reduction of the catalyst at 973 K results in formation of supported metallic Pt particles in SMSI (strong metal-support interaction) state, as evidenced by the sharp decrease of Pt ability to chemisorb H, at room temperature as well as the significant lowering of thermal stability of linear Pt-CO complexes. Spectroscopically the effect of SMSI is manifested by the slight increase of the singleton frequency of linearly adsorbed CO from 2050 to 2060 cm−1 together with the decrease of the dipole-dipole shift from 40 to 15 cm−1 and by a large decrease of absorption band extinction.

Diffuse-Reflectance IR Spectra of ortho-Hydrogen and para-Hydrogen Adsorbed on Zeolite BaX at 77 K and Their Interpretation in Terms of the Stark Effect

The vibrational and vibrational–rotational spectra of hydrogen adsorbed on zeolite BaX at 77 K were examined by diffuse-reflectance IR spectroscopy over a wide range of wavenumbers. The use of hydrogen enriched in the para isomer and of molecular deuterium allowed us to reliably assign the absorption bands to orthohydrogen and para-hydrogen and compare the experimentally observed band splitting with the theoretically calculated shifts of the corresponding levels resulted from the Stark effect. In the calculations, an adsorption site was simulated as a positive point charge. Although this approximation is rough, the IR spectra of adsorbed hydrogen and deuterium are adequately described in terms of the Stark effect. A decrease in the frequency of stretching vibrations of H–H (D–D) bonds in the molecules coordinated to Ba2+ cations, a decrease in the intensity of vibrational–rotational absorption bands, and the band splitting in the spectra of adsorbed para-hydrogen and orthohydrogen were explained.

Oxidized platinum surface in Pt/Al2O3 reforming catalysts: IR studies

The influence of halide ions on the state of dispersed Pt in oxidized catalysts has been investigated, adsorbed CO being used as a probe. Charged platinum complexes form upon catalyst oxygenation, with CO bands at 2130, 2150, 2165, 2170 and 2180 cm−1. Composition of the complexes depends on the oxidation temperature and the nature of the halide ion.

State of platinum in Pt/γ-Al2O3(Cl) reforming catalysts

Particular properties of metal (Pto) and electron deficient (Ptσ) platinum have been investigated by chemisorption and IR methods. Data about thermostability to sintering in hydrogen and to oxidizability are reported.

Comparative IR diffuse reflectance study of fundamental C=O stretching and combination C=O plus M-C stretching vibrational modes of CO linearly adsorbed on Ir/Al2O3 catalyst

Using diffuse reflectance IR spectroscopy, the comparative analysis of absorption bands corresponding to fundamental C=O stretching and combination C=O plus M-C stretching vibrational modes for12CO and12CO +13CO mixtures adsorbed on metallic Ir in supported 6% Ir/Al2O3 catalyst was performed. For CO adsorbed on metal in linear form the frequencies and intensities of combination absorption bands were insensitive to dipole-dipole coupling between adsorbed molecules. Therefore the carbon-metal bond stretching frequency could be calculated as the frequency difference of the combination mode and the singleton. For the linear form of carbon monoxide adsorption on supported Ir it is equal to 475 cm−1.